JAKARTA (Reuters) - Indonesian authorities on Tuesday said they had arrested the alleged financier and mastermind behind the hijacking of oil tanker Orkim Harmony in June, as well as two suspects in other piracy attacks.

Indonesia is stepping up its efforts against piracy, including the deployment of rapid reaction teams with neighboring Malaysia, to combat a soaring number of attacks on merchant vessels in one of the world's busiest shipping choke points.

The navy said authorities in Jakarta last week arrested a man identified only as "AJ", who was suspected to be involved in the Orkim Harmony hijacking.

"He admits to being the donor of operational funds and giving orders to take gasoline," it said in a statement.

Pirates reportedly armed with pistols and machetes seized the Orkim Harmony, which was carrying around 50,000 barrels of gasoline, near the Malaysian port of Tanjung Sedili. The ship, along with its 22 crew, was released a week later.

The navy said the suspect also admitted to having been behind four other pirate attacks, including those on the Everton and Danai-2 in 2012, besides two Vietnamese tankers.

In a separate raid, authorities arrested two individuals on the Indonesian island of Batam, as suspects in pirate attacks.

More than 70 ships have been attacked this year in the straits of Malacca and Singapore, on the western side of the Malay peninsula, the highest number since at least 2008, security and anti-piracy groups say.